The invention is directed to a process for the production of iminodiacetonitrile by reaction of hexamethylenetetramine with hydrogen cyanide in acidic aqueous medium.
It is known that iminodiacetonitrile is formed by the action of hydrogen cyanide on ammonia and formaldehyde. The reaction is carried out in aqueous medium using 3 moles of hydrogen cyanide to 2 moles of ammonia and 3 moles of formaldehyde at pH values of 5.5 to 6.5 (Miller U.S. Pat. No. 2,794,044) or using 1.0 mole of hydrogen cyanide to 2.2 to 4.0 moles of ammonia and 0.6 to 1.0 mole of formaldehyde at pH values of 7 to 9 (Saunders U.S. Pat. No. 3,167,580).
It is also known to produce iminodiacetonitrile from hexamethylenetetramine and hydrogen cyanide in acidic aqueous medium. For this purpose in discontinuous operation, there are used per mole of hexamethylenetetramine 6 moles of hydrogen cyanide and the reaction is carried out at a temperature of 0.degree. to 75.degree. C. with addition of weak acids in a buffer mixture at a pH of 5.0 to 6.5 (Stutts, U.S. Pat. No. 3,412,137) or there is added per mole of hexamethylenetetramine at least 6 moles of hydrogen cyanide and the reaction takes place with the addition of strong acids at a temperature of 30.degree. to 70.degree. C., whereby the materials are partially mixed with each other in several steps, so in such manner that the pH in the reaction is held at 5.5 to 6.5 (Cullen, German OS No. 2,613,994) or there are used in continuous operation per mole of hexamethylenetetramine 5 to 7 moles of hydrogen cyanide, the reaction mixture is adjusted to a pH of 2.0 to 6.9 by means of strong acids and the reaction carried out in a reaction tube at 50.degree. to 120.degree. C. whereby in a given case portions of the acids are fed in at several places in the reaction tube (Philbrook U.S. Pat. No. 3,886,198).
Besides it is known to produce iminodiacetonitrile by reacting an aqueous mixture containing for every 1.0 mole of hexamethylenetetramine 1.8 to 2.2 moles of glycolonitrile with 5.2 to 6.6 moles of hydrogen cyanide at a pH of 4 to 10 and a temperature of 50.degree. to 200.degree. C. (Gaudette U.S. Pat. No. 3,904,668) or by reacting an aqueous mixture containing for every 1.0 moles of hexamethylenetetramine 1.0 to 2.2 moles of formaldehyde with 6.9 to 8.6 moles of hydrogen cyanide while adding acid at a pH of 5 to 10 and at a temperature of 50.degree. to 250.degree. C. (Gaudette, German OS No. 2,639,874).
A disadvantage in all of the known processes is that the iminodiacetonitrile produced contains to a considerable extent undesired byproducts such as glycolonitrile, nitrilotriacetonitrile and methylene bis iminodiacetonitrile. These byproducts either cannot be separated from the iminodiacetonitrile or in all events can be separated only with considerable difficulty and loss of yields. They are disturbing in the further use of the iminodiacetonitrile, for example in the hydrolysis to iminodiacetic acid or in the hydrogenation to diethylenetriamine or piperazine.